Rotary hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more co-axial cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit.
Such hammers are provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive for driving a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beat piece.
Rotary hammers can be employed in combination impact and drilling mode, and also in some cases in a drilling only mode, in which the spindle, or a forward most part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. A rotary drive mechanism transmits rotary drive from the electric motor to the spindle to cause the spindle, or a forward most part thereof to rotate.
It is a problem with hand held power tools with a rotating bit that during use of the tool blocking events can sometimes occur, when the bit becomes stuck in the work piece in such a way that the bit can no longer rotate relative to the work piece. In this case, the rotary drive to the bit causes the housing of the tool to rotate with respect to the stationary bit. It is known to detect blocking events, for example from U.S. Pat. No. 5,914,882, U.S. Pat. No. 5,584,619, EP0771,619 and GB2,086,277 and then once such an event is detected, to arrest the rotary drive to the bit. This can be done by braking the motor, which motor provides rotary drive to the bit via a gear arrangement, but this involves an inherent delay due to the time required to arrest the motor. Alternatively or additionally, the rotating spindle can be braked by engaging the spindle with a part of the hammer housing, in some way. The preferred way of cutting off rotary drive to the bit is by the use of a releasable clutch arrangement in the drive train between the motor of the hammer and the spindle.
WO04/024398 describes a hammer drill which utilises a clutch arrangement to protect the hammer drill from such blocking events. Such clutches have torque fresh holds, whereby when a rotary torque is applied to the clutch which reaches a fresh hold, the clutch slips. WO04/024398 discloses a number of designs of clutch which have two torque fresh holds. The clutches can be adjusted between the two settings. This allows the clutches to be adjusted to suite the application for which the hammer is being used.
The present invention aims to provide an improved design of one of the clutches disclosed in WO04/024398.